The time of messages in Thunderbird always used to be annoying (i.e. non-24hrs format). This post explained what that’s due to: Thunderbird takes your default locale and presents time in the view due to that.
Somehow, the default locale it took was not my default locale (LC_ALL=C). So, to fix this minor annoyance, I changed the Thunderbird launcher to env LC_ALL=C thunderbird %u (where %u passes arguments — I just left it there).